Conventionally, various copolymers obtained by polymerizing a conjugated diene compound and an α-olefin have been known. For example, in Patent Document 1, it is disclosed to obtain a conjugated diene compound/α-olefin alternating copolymer by bringing butadiene into contact with an α-olefin in a liquid phase in the presence of a catalyst composed of an organoaluminum compound, vanadium (IV) chloride, and a compound having a carbonyl group or a cyano group.